Laura Marling, Soho Revue Bar, London

Audience members arriving to hear the British music scene's latest female prodigy were each handed a bag of Laura Marling jelly beans. When the show was over, the unworthy thought occurred that one day this small item of memorabilia might be worth quite a lot of money.

Such is the morally corrosive power of eBay on the modern psyche. Then again, Marling – performing on her 18th birthday – was so good that it was impossible not to see her becoming very big, very quickly. And the timeless nature of her talent – singer-songwriting in a mode that loosely evokes Joni Mitchell – suggests that exactly how long she stays at the top will be a matter entirely up to her.

It almost defied belief that music of this much assurance could emanate from one so young. Indeed, perhaps the greatest compliment Marling can be paid is to say that in admiring her one has no need to make any allowance for her age whatsoever. Her music – alt.folk, if it must have a label – can be judged against the highest adult standards and not be found wanting.

Let's make one other thing clear. Comparisons with Adele, Amy Macdonald, Duffy, Lily Allen, Kate Nash or any other gifted young British female artist you can think of are irrelevant. Sure, it's remarkable that all these people have come along at the same time, but beyond gender and youthfulness, Marling has little else in common with them.

It's true that she is something of a throwback, and there were times when her combination of poetic seriousness and melodic intimacy seemed to turn the Soho Revue Bar into a Greenwich Village coffee house circa 1961. But the music was never earnest, still less embarrassing, and a hushed crowd hung on her every word and note. They were invariably rewarded.

A slim figure in jeans and red ballet pumps, with a loose-fitting black top and very blond hair, Marling was charm personified when she chatted between numbers, but acquired a distant look in her eyes when she sang. Her lovely voice ranges from the delicate and breathy to the meaty and soulful, and will surely get even better in time.

Backing Marling's vocals and acoustic guitar were a bass-player, a violinist and drummer, but mostly what they provided could be described as promptings rather than arrangements. Marling's songs, which tend to deal in love and loss and are underpinned by a certain fatalism, are so strong and so flexible that they can largely stand on their own two feet.

She performed just nine of them, nearly all taken from her beguiling debut album Alas, I Cannot Swim, which is out this week. Lyrically dextrous, emotionally wary, they were as intelligent as they were listenable. The spare and affecting "Shine" ensured that Marling gripped from the start, before she upped the tempo with "Cross Your Fingers", which was adorned by a nice bossa nova closing section. The melancholic "My Manic and I" was another highlight, as was the striking conceit of the title track from her album: "There's a boy across the river, but alas I cannot swim. I will never get to put my arms around him."

She was called back for an encore but claimed she had none to offer. "I was so worried about getting in here tonight that I forgot to practise one," she said, a reference to when she was due to play at this venue last year but wasn't allowed to because at 17 she was too young (she ended up performing in the street outside).

Eventually, Marling and the band came up with a version of Kimya Dawson's "Five Years in the Saddle", which brought out the country singer in her, and then she was off to celebrate her coming of age. A brilliant future beckons.